Usually, power converter circuits are employed to transform certain input voltage waveforms into specified output voltage waveforms. In many occasions, the input DC voltages are transformed into desired output DC voltages, and the output voltages could be higher or lower than the input voltages. The typical applications of these converters include the power sources of communication systems and computers. Please refer to FIG. 1, it is the schematic circuit diagram of a typical full-bridge DC/DC converter in the prior art. In which, it includes four controllable switches (Q1 to Q4, usually MOSFETs), a power transformer (T1, usually an isolated transformer), output rectifier unit, and output filter (Lf and Co). There is also a controller for controlling the four controllable switches.
When the full-bridge DC/DC converter is operated, the two diagonal switches are alternately turned on and off according to specified duty ratio, and the AC voltage is added to the primary side winding of the transformer. When the AC voltage is transferred to the secondary side of the transformer and sent through the rectifying unit and the output filter, the AC voltage is transformed into the desired DC voltage. When the input DC voltage and the output current are changed, the controller are employed to monitor the output voltage, adjust the duty ratio of the two diagonal switches, adjust the amount of the rms of the AC component processed by the transformer, and accomplish the adjustment of the output voltage lastly.
However, it is quite easy to generate the volt-sec unbalancing phenomenon due to the deviation of the controller, and as a result the transformer would suffer from the DC magnetic flux. The volt-sec unbalancing means that there are DC voltage components added to the iron core of the transformer. The difference between the duty ratios of the control signals of the controllable switches and the unsymmetry of the voltage drops of the turned on controllable switches are factors which will cause the unbalancing of the volt-sec. After several periods of switching, continuously increased magnetic flux will cause the transformer to be saturated, and cause the power converter to become failed lastly.
The full-wave rectifier is usually employed as the output rectifier unit of the full-bridge DC/DC converter. The advantages of this method include that only one output inductor is required, and the DC component of the secondary side can be neglected. This method is widely used while the output current is not relatively quite large.
In the prior art, the method of coupling DC blocking capacitor to the primary side of the transformer is usually employed to solve the DC component problem of the full-bridge DC/DC converter as shown in FIG. 2. In which, Cb is the DC blocking capacitor, and the circuit of the secondary side is not shown. The principles of the aforementioned method are: 1. if the DC blocking capacitor Cb is not existed, there is DC component on the primary side of the transformer; and 2. if the DC blocking capacitor Cb exists, there is a DC voltage across Cb, and the effects of the DC component will trade off the DC current of the primary side. There is no DC current across the capacitor in the steady state. Otherwise, the capacitor voltage will be increased unlimited. Thus, there is no DC component on the primary side of the transformer coupled to the capacitor in series. The advantages of this alternative are: it is relatively simple, feasible, and more reliable. The disadvantages are: 1. the voltage across the capacitor Cb should be relatively smaller to keep the original features of the circuit, which means that the capacitance of the DC blocking capacitor should be relatively larger; 2. this will increase the complexity of the circuit and the relative costs of the system; 3. the volume of the system is increased, and hampered the increasing of the power density of the converter. Surely, the capacitance of that capacitor could be decreased properly under certain application occasions. But for these certain occasions, the DC blocking capacitor has other negative influences. For example, the DC voltage across the DC blocking capacitor of the commonly used phase-shifted full-bridge DC/DC converter will cause the inconsistence of the soft-switching conditions of the circuit according to the analyses. Under certain conditions, it is possible that the system would be damaged due to the over-heating.
Except for employing the DC blocking capacitor to remove the DC component on the primary side of the transformer, there is another alternative which is feasible theoretically. Since the DC component will cause the transformer to be saturated due to the magnetic flux, the result caused by the saturation of the transformer will be the dramatically increasing of the current on the primary side, and the power switch on the primary side will be damaged lastly. Therefore, a feasible alternative is to increase the air gap of the transformer, thus the transformer can stand relatively larger DC current and could not be saturated easily, and the switches of the circuit will not be damaged due to the over-current. But the introduce of the air gap will decrease the exciting inductance of the transformer, and increase the exciting current. This will cause the copper losses on the primary side of the transformer to be increased, which is equivalent to that the AC impedance of the primary side winding is increased. Finally, the efficiency of the converter is relatively low. It can be seen that the relatively less air gap is desired for the higher efficiency, but the relatively less air gap will cause the transformer to be saturated more easily.
Keeping the drawbacks of the prior arts in mind, and employing experiments and research full-heartily and persistently, the methods and controllers for suppressing DC magnetic deflection of transformer are finally conceived by the applicants.